Acanthodians

Acanthodians are one of the large groups of fish that did not survive the passage of time, having disappeared during the Permian Period.(52 kb) Nonetheless, their presence was hardly fleeting. Appearing at the end of the Ordovician Period and the beginning of the Silurian, acanthodians swam the worlds waters for 150 million years.

Like the placoderms, we only know acanthodians in their fossilized form. The name comes from the Greek acanthus, meaning spine, for the hard spine in front of each of their fins. The fins of an acanthodian are quite distinctive because they lack rays, consisting instead of small membranes of skin covered with the same scales that cover its body. Acanthodians are also distinguished by their very small diamond-shaped scales, somewhat similar to those of sharks.

Acanthodians might well be the oldest gnathostomes in the fossil record. Acanthodian-like scales were found in Ordovician layers, and fragmentary fish fossils are present in rocks from early Silurian time. They prospered mainly in the Devonian Period, during which time they attained worldwide distribution, although never becoming the predominant group in any given region.

Researchers recognize three major groups of acanthodians divided into roughly sixty genera for a total of 150 different species. Many of these species, however, are known only from their spines, teeth or small scales because fossilization of their skeletons, made of fragile cartilage, is quite rare and often incomplete if it occurs, hence the great significance of the four Miguasha specimens.

Most acanthodians grew no longer than 25 cm, although Gyracanthus reached up to two meters during the Carboniferous Period. Their bodies were long and trapezoidal, their mouths subterminal, their tail heterocercal, and their eyes quite large and positioned far forward on their heads. This morphology suggests a well developed sense of sight and a lifestyle characterized by active swimming. Some were carnivores, with many small teeth. Those without teeth, including the four Miguasha species, filtered their food while swimming. Their spines must have served as a good defence against predators, although their remains are sometimes found in the abdomens and coprolites of other species.